Radar (which is an acronym for RAdio Detection And Ranging) is a technique used to detect objects at a distance through the transmission of electromagnetic energy (usually at RF or microwave frequencies). Radar systems are used for a wide variety of purposes, including meteorological purposes (e.g., the NEXRAD system), e.g., to detect storm systems. Radars are used on board planes and ships to detect and track objects, both on the surface and in the air, for both military and commercial applications. Examples of such systems include AN/SPQ-9(b) naval radar system available from Northrop Grumman Norden Systems, Inc., which is used to detect surface vessels and small, low-flying objects such as missiles, and the Pathfinder commercial shipboard radar system available from Raytheon Corp., which is used to detect other ships and for navigational purposes. Radars are also used for collision avoidance on automobiles. It is also known to use radars for imaging, both for celestial and terrestrial objects. For example, the Joint Surveillance Target Attack Radar System (JSTARS) uses a synthetic aperture radar (SAR) mounted on the underside of a converted 707 airframe to form ground images which can reveal the presence of military vehicles such as tanks.
Radar systems can be either continuous wave or pulse types. In both types of systems, a portion of the energy transmitted toward an object by a transmitting antenna is reflected toward a receiving antenna to provide information about the object. In a continuous wave radar system, electromagnetic energy (typically at RF or microwave frequencies) is continuously transmitted, while in pulse radar systems, electromagnetic energy is transmitted in short bursts, or pulses, at a frequency referred to as the pulse repetition frequency, or prf The prf is often chosen based on the maximum expected range at which target detection is desired. This range is referred to as the unambiguous range. In such pulse systems, the pulses have a period, or pulse width, which is typically short as compared to the period between the pulses (which is the inverse of the prf). After a pulse is transmitted, a receiver “listens” for echoes of the transmitted pulse reflected by an object. If an echo is received, the object has been detected.
In pulse systems the range of an object can be determined by determining the time between the transmitted pulse and receipt of the echo (assuming that the range is less than the unambiguous range). If the transmitting and/or receiving antenna (the same antenna is often used for both functions) has directional properties, information concerning the bearing of the target may also be revealed by the echo.
Radar may also be used to determine relative velocity of an object with respect to the radar system. For example, in Doppler radar systems, frequency and/or phase shifts in the echo can reveal the relative velocity between object and system. In such pulse radar systems, echoes received from several pulses are often “batched” or combined (e.g., added and averaged). The echoes from a plurality of pulses that are combined in this fashion are often referred to as a batch. In a surveillance radar application, in which a rotating directional antenna with a beam width on the order of 5-10 degrees and a rotation rate of on the order of 1-10 seconds is utilized, batches of 2-16 pulses are typical, whereas batches of 50-250 pulses are more common to high resolution radar systems with non-rotating antennas, such as the type utilized in automobiles for collision avoidance. Batching serves to both increase the signal-to-noise ratio of the echo and to decrease the amount of processing required.
With radar systems, like any communication system, it is desirable to transmit information at the lowest cost and highest speeds possible. Radar systems, again like other communications systems, are also subject to degradation in performance resulting from interference and noise. This is especially true for radar systems such as collision avoidance radar systems on automobiles, in which each radar transmitter/receiver in the system is subject not only to interference from other transmitters in the system and/or on the vehicle, but also is potentially subject to interference from multiple other vehicles in the vicinity. Improving the performance of radar systems in the face of interference and noise has long been a goal of radar system engineers. While much progress has been made over the 60 or so years since radar has been in existence, improvement is still desirable and necessary.
What is needed is a system and method for reducing the effects of interference and noise in a radar system.